1. Field of the Invention
The present invention relates to an image forming apparatus using an electrophotographic method, and more particularly to an image forming apparatus such as a copying machine, a printer or a facsimile machine.
2. Related Background Art
FIG. 8 shows a schematic structure of an example of a process station (image forming station) in a conventional image forming apparatus using the electrophotographic method.
A process station 2 shown in FIG. 8 includes a photosensitive drum 21 that rotates in a direction indicated by an arrow R2 as an image bearing member. After the surface of the photosensitive drum 21 is uniformly charged by a primary charger 22, the surface receives an exposure 23xe2x80x2 based on image information by an exposure device 23 such as an LED or a laser, to thereby form an electrostatic latent image thereon. Toner is stuck onto the latent image by a developing sleeve 27 of a developing device 24 which rotates in a direction indicated by an arrow R3, to thereby develop the latent image as a toner image. The toner image thus obtained is electrostatically transferred by a transfer charging blade 103 onto a transfer material P which is born on a conveying belt 1 and conveyed in a direction indicated by an arrow R1.
After the toner image has been transferred onto the transfer material P, non-transferred toner which has remained on the surface of the photosensitive drum 21 without being transferred onto the transfer material P is removed by a cleaning blade 25 and then collected within a waste toner container 26. The photosensitive drum 21 the surface of which has been thus cleaned is ready for succeeding image formation.
FIG. 9 shows a four-color full color image forming apparatus of a so-called in-line system in which four process stations having the same structure as that of the above-described process station 2 are arranged.
As shown in FIG. 9, in the image forming apparatus, a conveying belt 1 is disposed as transfer material conveying means, and the conveying belt 1 is put around four rollers consisting of an attractive opposite roller 6, a driving roller 7 and tension rollers 8 and 9, and rotated in a direction indicated by an arrow R1 by the drive of the driving roller 7. Four process stations 2a, 2b, 2c and 2d for yellow (Y), magenta (M), cyan (C) and black (K) are disposed tandem in the stated order from the upstream side along the moving direction of the conveying belt 1 as indicated by the arrow R1.
Transfer blades 103a, 103b, 103c and 103d are abutted against the conveying belt 1 in order to make the conveying belt 1 push with predetermined pressure toward photosensitive drums 21a, 21b, 21c and 21d within the respective process stations 2a, 2b, 2c and 2d. The respective transfer blades 103a, 103b, 103c and 103d are connected with transfer bias power supplies 104a, 104b, 104c and 104d each of which applies a transfer bias.
Up to now, the photosensitive drum 21 (21a, 21b, 21c and 21d) is made of negative organic photoconductor (OPC), and in the case where an exposed portion of the surface of the photosensitive drum 21 where negative electric charges are decayed by the exposure of the exposing device 23 is developed, a developer containing negative toner therein is employed. Accordingly, during transferring operation, a positive transfer bias is applied to the transfer blade 103 (103a, 103b, 103c and 103d) by the transfer bias power supplies 104 (104a, 104b, 104c and 104d). The transfer blade 103 is generally formed of a low-resistance resin film.
The transfer material P is conveyed from a sheet feed cassette 15 toward the conveying belt 1 by a feed roller 14. In order to make the transfer material P in synchronism with the toner image formed on the photosensitive drum, after the transfer material P thus conveyed is nipped by a pair of registration rollers 10 and 11 once, the transfer material P is conveyed in predetermined timing to an attractive portion N of the conveying belt 1 where an attractive roller 5 and an attractive opposite roller 6 are opposite to each other, and the transfer material P is attracted to the conveying belt 1 by the attractive roller 5 and the attractive opposite roller 6.
A given voltage is applied to the attractive roller 5 from an attractive bias power supply 12 which is a high voltage power supply, as a result of which electric charges are induced to the transfer material P, and the conveying belt 1 is dielectrically polarized, thereby allowing the transfer material P to be electrostatically attracted onto the surface of the conveying belt 1 so as to be firmly born by the surface of the conveying belt 1.
The transfer material P born on the conveying belt 1 sequentially passes through the respective transfer portions of the four process stations 2, and the toner images of the respective colors on the respective photosensitive drums 21a, 21b, 21c and 21d are sequentially superimposed and transferred on the transfer material P by the action of the transfer blade 103, to thus obtain a full color image of four colors on the transfer material P.
The transfer material P onto which the toner image of four colors has been thus transferred is separated from the conveying belt 1 and is then conveyed to a fixing device 16 where the transfer material P is heated and pressurized so that the toner image is fixed on the surface of the transfer material P, to thereby form a permanent image of the full color. Thereafter, the transfer material P is discharged to the external of the image forming apparatus. After electric charges are eliminated by a charge eliminating charger 13 from the surface of the conveying belt 1, from which the transfer material P has been separated, the conveying belt 1 is ready for a succeeding image forming process.
Up to now, the conveying belt 1 is formed of a resin film such as PVdF (polyvinylidene fluoride), ETFE (tetrafluoroethylenexe2x80x94ethylene copolymer), polyimide, PET (polyethylene terephthalate) or polycarbonate which is 50 to 200 xcexcm in thickness and about 109 to 1016 xcexa9cm in volume resistivity, or an urethane rubber in which fluorine resin such as PTFE is dispersed coated on a base layer made of rubber such as EPDM which is about 0.5 to 2 mm in thickness.
There is no case in which the conveying belt 1 is contaminated by the toner since the toner image is not directly transferred onto the surface of the conveying belt 1 during the normal image forming operation. However, in the case where the transfer material P is jammed or fogging toner on background is stuck onto a non-image portion of the photosensitive drum, the conveying belt 1 is contaminated by the toner. Also, in the case of executing a density control mode that controls the density of a toner image formed on the photosensitive drum or a registration control mode that controls a timing at which the toner images of the respective colors are formed on the respective photosensitive drums so as to appropriately superimpose the toner images of the respective colors on the transfer material P on each other, since a density patch or a register patch is transferred onto the conveying belt 1 as a detection toner image and then detected by a photosensor 31, the conveying belt 1 is contaminated by the toner by the density patch or the register patch.
The contamination toner on the conveying belt 1 is cleaned and removed by the cleaning blade 32 which is disposed opposite to a backup member 33 through the conveying belt 1. The removed toner is collected into a waste toner container 34.
Since the waste toner container 34 needs to be replaced by a fresh container when the container 34 is filled with the collected toner, this replacing work is troublesome for a user, as a result of which the useability may be lowered. Also, since the waste toner container 34 is so designed as to be replaceable, there arises such a problem that the structure of the main body of the image forming apparatus is complicated.
If the waste toner container 34 is made larger in sized in order to improve the useability, the main body of the apparatus is caused to be large in size in any case where the waste toner container 34 is fitted to a conveying belt unit having the conveying belt 1 or the main body of the apparatus.
In order to solve the above drawbacks, there has been proposed a cleaning system in which a cleaning bias is applied to the transfer blade 103 with no provision of a dedicated belt cleaner on the conveying belt 1 to electrostatically collect the contamination toner on the conveying belt to the photosensitive drum 21. In the image forming apparatus of the in-line system, since four photosensitive drums 21 are provided, there are four chances where the contamination toner on the conveying belt is collected, resulting in a very advantage.
When the contamination toner is removed, if the polarity of a voltage which is applied to the transfer blade 103 is made different among the respective process stations 2 to form a counter electric field in each of the stations, both of the positive contamination toner and the negative contamination toner can be collected, or if a supply voltage is made large to electrically charge the contamination toner on the conveying belt at a transfer portion, the contamination toner can be collected in a succeeding process station.
Up to now, the electric field cleaning operation is conducted at a timing different from that of a normal image forming sequence, for example, in a period between a time the image forming operation is completed and a time the image forming apparatus is suspended.
However, in the above-described image forming apparatus, when image formation is continuously conducted on a plurality of transfer materials, there may occur such a drawback that the hue or tone of a toner image (a change in density) formed on each of the transfer materials is changed, or misregister, that is, out-of-color registration occurs in the toner image of each color which is formed on each of the transfer materials.
Under the above circumstances, in order to solve the above drawback, an attempt has been made to form a density patch or a register patch on the conveying belt 1 between the adjacent transfer materials (so-called a space between sheets) even while the image formation is sequentially conducted on a plurality of transfer materials to execute the above-mentioned density control mode or the registration control mode. As a result, there occurs the following different problem.
In the image forming apparatus as shown in FIG. 9, since the density patch or the register patch formed on the conveying belt 1 is cleaned off by the cleaning blade 32 which is always abutted against the conveying belt 1, even in the case of continuously forming an image on the transfer material subsequent to the density control mode or the registration control mode, there is no case in which a back surface of the transfer material is contaminated by the patch toner image.
However, in the case where the above-mentioned electric field cleaning system is executed in a succeeding round of the conveying belt after the density patch or the register patch formed in a space between sheets on the conveying belt 1 is detected by a sensor 31 without using a mechanical cleaning system such as the cleaning blade 32 shown in FIG. 9, the density patch or the register patch on the conveying belt 1 is conveyed to the attractive portion N or the transfer portion of each process station. Therefore, in the case where the image is continuously formed on the transfer material subsequent to the density control mode or the registration control mode, there occurs such a problem that the back surface of the transfer material is contaminated by the patch toner image. In addition, this leads to such a problem that the attractive roller 5 is contaminated by the patch toner.
The present invention has been made to solve the above problems, and therefore an object of the present invention is to provide an image forming apparatus which controls a timing at which a transfer material is supplied to a transfer material bearing member so as not to superimpose a detection toner image on the transfer material, thereby being capable of preventing the back surface of the transfer material from being contaminated.
Another object of the present invention is to provide an image forming apparatus which controls a timing at which a toner image which is to be formed on the transfer material so as not to be superimposed on the detection toner image is formed on an intermediate transfer member, thereby being capable of preventing an image failure from occurring.
Other objects of the present invention will become apparent from the following detailed description.